I think the most common thing about the Halogens and the Alkali metals is their chemistry of gaining or losing electron. In fact Alkali metals come in the first group and they have only one electron in their outermost shell. On the other hand Halogens come in the seventh group of the periodic table and they have seven electrons in their outermost shell.
According to the law of octet or duplet, an atom of an element had to complete either 8 or 2 electrons in its outermost shell.
Therefore, Alkali metals like Li, Na, K, Rb, Cs, and Fr lose one electron from their outermost shell and gets the valency of +1, however, Halogens like Fl, Cl, Br, and Iodine get an electron to complete outermost shell and get a valency of - 1. In this way both have same valence number but only one gains and other loses. That is also one of the major reasons for their attraction for each other.
According to the law of octet or duplet, an atom of an element had to complete either 8 or 2 electrons in its outermost shell.
Therefore, Alkali metals like Li, Na, K, Rb, Cs, and Fr lose one electron from their outermost shell and gets the valency of +1, however, Halogens like Fl, Cl, Br, and Iodine get an electron to complete outermost shell and get a valency of - 1. In this way both have same valence number but only one gains and other loses. That is also one of the major reasons for their attraction for each other.
